1. Field of the Invention
The present invention relates to a microetching composition useful for treating surfaces of copper or copper alloys (the copper or copper alloys may be hereinafter collectively called "copper").
2. Description of the Background Art
In the manufacture of printed wiring boards, copper surfaces are polished prior to coating such surfaces with an etching resist or a solder resist to improve adhesion of these resists to the copper surfaces. Mechanical cleaning using a buffing or scrubbing machine, or chemical cleaning, known as microetching, are used for roughening the copper surfaces. Microetching is more popular for treating substrates with fine line patterns. The copper surface is etched as deeply as 1-5 .mu.m by the microetching treatment. This depth of etching is calculated from the weight of copper decreased by etching, the specific gravity, and the surface area of the copper.
The microetching is also applied to remove oxidized films from the surface of copper prior to a solder leveling process or prior to mounting electronic parts in order to improve solderability. An aqueous solution containing sulfuric acid and hydrogen peroxide as major components or an aqueous solution containing persulfate as a major component is used for the microetching.
As the circuit density of printed wiring boards become higher, conventional heat curable solder resists are being replaced by UV curable solder resists which are superior in forming fine line patterns. The characteristics of the UV curable solder resists in adhering to copper surfaces, however, are weaker than those of conventional solder resists. The copper surfaces obtained by conventional microetching therefore exhibit insufficient adhesion to these UV curable solder resists. Resist films may be peeled or may swell during the subsequent steps of gold plating, solder leveling, or electronic part mounting.
The solderability of copper surfaces obtained by conventional microetching is also insufficient for the solder leveling process for printing wiring boards which has an increasing number of pads for surface mount devices. Soldering failures sometimes occur.
Therefore, development of a microetching agent which can produce roughened surfaces on copper or copper alloy with adequately deep irregularities exhibiting excellent adhesion of solder resists and the like has been desired.
The inventors of the present invention have undertaken extensive studies and found that the addition of an unsaturated carboxylic acid, a salt thereof, or an anhydride thereof to a microetching agent comprising nitric acid or sulfuric acid and ferric nitrate or ferric sulfate, can achieve this object.